rename detection with -M100 means "exact renames only".

rename detection with -M100 means "exact renames only".

When the user is interested in pure renames, there is no point
doing the similarity scores.  This changes the score argument
parsing to special case -M100 (otherwise, it is a precision
scaled value 0 <= v < 1 and would mean 0.1, not 1.0 --- if you
do mean 0.1, you can say -M1), and optimizes the diffcore_rename
transformation to only look at pure renames in that case.

Signed-off-by: Junio C Hamano <junkio@cox.net>